This invention relates to improved polyamide fibers having enhanced luster and dye light-fastness properties and acceptable spinnability characteristics, and to processes for producing such fibers.
In general, unadulterated melt-spun polyamide fibers are relatively transparent, with a "bright", shiny or sparkling appearance. To achieve soil-hiding capability for such applications as carpets, various adulterants have been added to polyamides during polymerization or melt-spinning steps. Such adulterants impart opacity to spun fibers, which in turn provides the desired soil-hiding characteristics. For example, pigments such as titanium dioxide have been added for this purpose, in a process known as delustering. However, delustering with titanium dioxide decreases surface luster, resulting in a dull or chalky fiber finish.
The achieve a lustrous fiber with opacity, various methods have been developed for creating a multiplicity of longitudinal microscopic internal voids, typically disposed parallel to the fiber axis. These voids reflect incident light in a directional fashion (as opposed to the random scattering obtained when fibers are delustered with titanium dioxide), resulting in an opaque fiber with an aesthetically desirable silk-like luster. One method for creating such voids involves dispersing a water-soluble polymeric additive in a polyamide melt prior to melt-spinning. After spinning, a significant portion of the dispersed additive is extracted in a subsequent process step, resulting in void formation. The following references represent various adaptations of this method.
Magat et al., U.S. Pat. No. 3,329,557, disclose antistatic filaments of melt-spun synthetic linear polymers, e.g., polyamides, containing at least 2% by weight of a poly(alkylene ether) having an average molecular weight from about 600 to about 3,000,000. This additive is uniformly dispersed in the polymer melt prior to spinning, and can be partially extracted in an aqueous scouring step to achieve some void formation. A residue of the additive remains after scouring, which provides the anti-static properties. According to this reference, preferred additives for polyamide fibers are poly(ethylene ether) glycols having an average molecular weight from about 10,000 to 500,000, which are present in amounts ranging from 3% to 15% by weight.
Magat et al., U.S. Pat. No. 3,475,898, disclose static-resistant melt-spun polyamide fibers, containing as a distinct phase at least 2% by weight, based on polyamide, of a high molecular weight water-soluble poly(alkylene ether). In a preferred embodiment, between 3% and 15% by weight of a water-soluble poly(alkylene ether) glycol of average molecular weight from 1,000 to 30,000 is added to polyamide melts prior to spinning.
Etchells, U.S. Pat. Nos. 4,052,493 and 4,091,022, discloses polyamide fibers comprising between 1% and 14% by weight of an additive produced by reaction of boric acid and a poly(oxyalkylene) material having an average molecular weight of at least 600. According to this reference, addition of borate derivatives of poly(oxyalkylene) materials to polyamide fibers tends to eliminate certain detrimental effects upon dye light-fastness associated with poly(oxyalkylene) additives.
Kato et al., Japanese Patent No. 645,900, disclose anti-static polyamide fibers comprising at least 1% by weight of a mixture of poly(alkylene ether) materials of varying molecular weight. Specifically, this reference discloses additive mixtures containing between 10% and 70% by weight of poly(alkylene ether) compounds having 40 moles or less of alkylene oxide adduct units (implying a molecular weight up to about 1760) in combination with between 90% and 30% by weight of poly(alkylene ether) compounds having 100 moles or greater of alkylene oxide adduct units (implying a molecular weight greater than about 4400). According to this reference, such additive mixtures impart anti-static and water-absorbing qualities to polyamide fibers.
The foregoing methods of achieving void formation in melt-spun polyamide fibers, while useful, are complicated by considerations relating to the manufacturing process. Generally, poly(alkylene ether) compounds of low molecular weight are easily extracted from fibers in an aqueous scouring step, resulting in void formation. In practice, however, addition of more than 1% by weight of such materials significantly reduces the viscosity of polyamide melt-spinning mixtures, degrading the spinnability characteristics of the mixtures. Poly(alkylene ether) compounds of high molecular weight do not degrade viscosity, but are relatively more difficult to extract from spun fibers, thus negatively affecting void formation. In addition, residues of poly(alkylene ethers), upon standing, deleteriously affect dye light-fastness properties.
Addition of conjugates of boric acid and poly(alkylene ethers) partially alleviates the foregoing difficulties, since such materials are readily extractable from spun fibers, improving dye light-fastness. However, such materials are susceptible to hydrolysis as a result of absorption of atmospheric moisture. If these materials are not prepared, stored, and used in a manner designed to preclude contact with the atmosphere, their use can significantly decrease the viscosity of polyamide melts, degrading fiber spinnability.
Accordingly, improved polyamide fibers incorporating stable additives which provide a silk-like luster, acceptable dye light-fastness, a minimum of manufacturing complications, and uncompromised spinnability characteristics are of interest to the fiber and textile industries.